Crushing Blow

In June 1997, the Near Earth Asteroid Rendezvous spacecraft flew past
the asteroid 253 Mathilde, sending back images of a crater-battered world
about 52 kilometres in diameter, with five giant craters each over 20 km
in diameter. Craters of such a size are generally surrounded by blankets of
ejected material several kilometres deep, but on Mathilde there are no
signs of such material.

The asteroid's unusually low density is thought to be part of the
explanation for this lack of ejected material, and this is now confirmed
by hypervelocity impact experiments carried out by Kevin R. Housen
of The Boeing Company, Seattle, Washington, and colleagues. These
experiments -- which involve firing projectiles at very high velocities
at samples of a porous material -- suggest that the craters are produced
by compaction, rather than excavation. Such a compaction process would
result in relatively little ejected matter being lost into space, explaining
why material from highly porous asteroids is a rarity in meteorites
reaching Earth.

Erik Asphaug of the University of California at Santa Cruz, California
discusses these findings in an accompanying News and Views article.

Crushable Asteroids May Absorb Impacts

Some may resemble squeezable slabs of Styrofoam instead of brittle, dense
rocks that shatter on impact, according to a new study. Planetary scientists
have found striking similarities between craters blasted in soft soils in
the lab and the pattern of gouges on an asteroid named Mathilde. They
suggest that some asteroids absorb energy from collisions, growing smaller
and denser over time rather than breaking apart as other cosmic objects
pummel them.

First close-up mug shots of asteroid 253 Mathilde published

Images reveal a piece of heavily pocked cosmic real estate

ITHACA, N.Y. -- So many craters, so little asteroid.

Cornell University astronomer Joseph Veverka and a team of scientists
are releasing the first close-up images of a little-known C-class asteroid,
253 Mathilde, to be published exclusively in the journal Science (Dec.
19). Until now, astronomers have been able to do little but gaze through
telescopes and observe the minor planet, discovered 112 years ago. On
June 27 of this year, the Near Earth Asteroid Rendezvous (NEAR)
spacecraft passed within 1,212 kilometers of Mathilde and took images of
the asteroid. Scientists didn't expect to find the minor planet so densely
pocked with craters and so porous, as it is made mostly of carbonaceous
chondrite.

"Mathilde is very porous, and we still don't know if it was formed that way
originally," said Veverka. "This is the first time anyone has ever looked at
an asteroid like this and we were surprised at how 'underdense' it is on
the inside."

After reviewing 534 frames of images taken with a variety of equipment
during the close flyby in June, scientists were surprised to find so many
large craters packed so tightly on the relatively small surface of Mathilde.
This means that large objects have been able to strike the asteroid's
surface without destroying it, Veverka said. "Hitting Mathilde is like
hitting a Styrofoam cup or packing material," he said.

"Even more remarkable than the simple existence of these large craters
is the degree to which their rim crests and basic shapes seem minimally
affected by subsequent large impacts," scientists write in the article
describing the images, "NEAR's flyby of Mathilde: Images of a C Asteroid."
The article's authors include Veverka and Cornell scientists Peter Thomas,
senior research associate; Ann Harch, research support specialist; Beth
Clark, research associate; James F. Bell III, senior research associate;
Brian Carcich, systems programmer, and Jonathan Joseph, programmer,
all in the astronomy department. In addition to the Cornell members of the
NEAR team, astronomers from the Southwest Research Institute, Boulder,
Colo.; Northwestern University; Space Science Systems, San Diego,
Calif.; the University of Maryland, College Park; and Johns Hopkins
University's Applied Physics Laboratory participated in the image analysis
and the writing of the paper.

Currently, the NEAR spacecraft is on its way to the S-class asteroid 433,
named Eros. Enroute to Eros, NEAR flew by Mathilde on June 27 to gather
information. At a flyby speed of 9.93 kilometers a second, the spacecraft
spent about 25 minutes relatively close to the asteroid. During its closest
approach, the spacecraft took 144 high-resolution images of the minor
planet's irregular shape and heavily cratered surface.

The slow-rotating Mathilde resides in the asteroid belt, which contains
thousands of minor planets between Mars and Jupiter. At the time the
images were taken, Mathilde was approximately 203 million miles from
Earth. Mathilde rotates once every 17.4 days; only two other known
asteroids rotate more slowly: 288 Glauke and 1220 Crocus.

"That's one of the other mysteries of Mathilde," said Veverka . "It is so
sluggish and we still don't know why."

Launched aboard a Delta II rocket on Feb. 17, 1996, the NEAR spacecraft
mission cost NASA about $122 million and is the first of NASA's Discovery
Missions, which include a series of small-scale spacecraft designed to
proceed from development to flight in under three years for a cost of less
than $150 million each. In February 1999, the spacecraft will rendezvous
with Eros and orbit that minor planet for about a year.